It is well known that maximum activity of the Group VIII noble metals for hydrogenation reactions depends upon maintaining the metal in a finely divided state such that there is a maximum ratio of surface area to mass. Perhaps the most common method of achieving a high degree of dispersion involves impregnating salts of the Group VIII noble metals upon porous solid supports, followed by drying and decomposing of the impregnated salt. On non-zeolitic supports, the drying and calcining operations often bring about a substantial migration and agglomeration of the impregnated metal, with resultant reduction in activity. In more recent years, with the advent of highly active crystalline zeolite catalysts of the aluminosilicate type, it has become common practice to ion-exchange the desired metal salt into the zeolite structure in an attempt to achieve an initial ionic bond between each metal atom and an exchange site on the zeolite, thus achieving the ultimate in dispersion of metal while also bonding the metal to the zeolite in such manner as to minimize migration and agglomeration during the drying and calcining steps, in which at least a portion of the metal is oxidized and converted to a non-zeolitic form. This ion exchange technique is particularly desirable in the case of dual-function catalysts such as hydrocracking catalysts wherein it is desirable to maintain an active hydrogenating site closely adjacent to an acid cracking site. These efforts have met with varying degrees of success.
Even though the above described ion-exchange techniques can give a high degree of initial dispersion of the Group VIII noble metal on the support, conditions encountered during subsequent use of the catalyst may bring about a maldistribution of the metal with resultant reduction in activity, entirely independent of normal deactivating phenomena such as coking, fouling, poisioning, etc. Overheating, or contact with excessive partial pressures of water vapor at high temperatures, such as may occur during oxidative regeneration of the catalyst or during prolonged contacting with hydrocarbon feedstocks, may bring about migration of the active metal away from the exchange sites, and this migration may ultimately result in agglomeration of the metal, or at least migration to sites unavailable to feed molecules.
Migration and agglomeration are most apt to occur when the catalyst, in a sulfided condition (as e.g., in normal use for hydrocracking), or in an oxidized state (as during regeneration), comes into contact for more than about 30 minutes with water vapor of greater than about 3 psi partial pressure at temperatures above about 500.degree. F. The process of this invention is designed to effect redispersal of such agglomerated metal, and to achieve at least a complete recovery of fresh activity; but in nearly all cases it is found that the rejuvenated catalysts actually exhibit greater than fresh activity.
In the case of catalysts which originally contained a difficultly reducible zeolitic monovalent and/or divalent metal such as sodium, calcium, magnesium, nickel, manganese or the like, it has been found that the above described conditions encountered during use of the catalyst also appear to bring about a detrimental redistribution of the zeolitic metal cations. Residual zeolitic metal cations, particularly sodium, are believed to occupy mainly the relatively unavailable exchange sites in the hexagonal prisms and sodalite cages of the original zeolite structure, but under the described conditions of use, migration to more active cracking sites appears to occur with resultant loss in cracking activity. Divalent metal cations such as the alkaline earth metals which may have been originally exchanged into the zeolite to achieve hydrothermal stability, may also migrate to undesirable sites. It is hence desirable in the case of these damaged catalysts to remove at least some of the zeolitic mono- and/or divalent metal cations, in addition to redistributing the non-zeolitic Group VIII noble metal hydrogenating component. These are the major objectives of the present invention.
As employed herein, the term "non-zeolitic metal" refers to the metal content of the catalyst, other than anionic lattice metals such as aluminum, which is not chemically bonded to the anionic exchange sites of the zeolite, while conversely, "zeolitic metal" refers to the metal content which is so bonded. The easily reducible metals such as the Group VIII noble metals are normally present primarily as non-zeolitic metal, as a result of previous reduction with hydrogen, oxidation and/or sulfiding treatments. The difficulty reducible metals such as the alkali and alkaline earth metals are normally present almost exclusively as zeolitic cations, since they are not affected by the usual reduction, oxidation or sulfiding treatments. Metals of intermediate reducibility such as nickel, copper and the like may be present in both zeolitic and non-zeolitic form.
In broad aspect, the rejuvenation procedure of this invention involves two basic steps. In the first step the damaged catalyst is soaked in a volumetric excess of aqueous ammonium hydroxide, with time and temperature conditions adjusted to effect the desired redistribution of the Group VIII noble metal. This restores hydrogenation activity, but does not appreciably restore lost cracking activity, unless high temperatures are employed, sufficient to exchange out some of the zeolitic meal content. At ambient temperatures, ammonium hydroxide extracts little if any zeolitic cations such as sodium or magnesium.
In the second step of the process, the catalyst is treated with extractive proportions of an aqueous ammonium salt solution under neutral or slightly acidic conditions, whereby a substantial porportion of zeolitic metals such as the alkali or alkaline earth metals are exchanged out and replaced by ammonium ions. This is believed to be the effective mechanism for restoring the cracking activity of the catalyst.
Following the first step of the process, it is necessary to strip substantially all ammonia from the catalyst in order to avoid encountering an alkaline pH in the second step; under alkaline conditions it is difficult to exchange out zeolitic metal cations with ammonium ions. The ammonia can be removed after the first step by water washing, but only with difficulty, for the acidic zeolite tends to strongly retain ammonia. An intermediate drying and calcining procedure to drive off ammonia would add greatly to the cost of the process.
To avoid these difficulties I have found it to be most expeditious to simply add sufficient acid to the ammonium hydroxide-catalyst mixture from the first step to neutralize the ammonia, thereby providing the desired ammonium salt for the second step, as well as the desired pH. This procedure also provides the fortiutous advantage of greatly reducing the amount of spent ammoniacal reagents produced, which in the past have created a serious problem of disposal in a manner consistent with environmental control standards set by governmental agencies.
A surprising aspect of the invention is that the aqueous ammonium hydroxide solution used in the first step does not extract any significant amount of the Group VIII noble metal from the catalyst. In copending application Ser. No. 321,670, filed Jan. 8, 1973, a progenitor rejuvenation process is disclosed, which involves treating the damaged catalyst with gaseous ammonia and water vapor under controlled conditions of hydration. This treatment was found to be sufficient to effect desirable redistribution of the Group VIII noble metal, resulting in a substantial recovery of hydrogenation activity, but since extractive proportions of the ammoniacal reagent were not employed, there was no possibility of concomitant removal of zeolitic metal cations. It was believed at the time that the presence of excess aqueous ammonia would tend to solubilize the Group VIII noble metal as amminohydroxide which would then be leached out of the catalyst. It hence came as a distinct surprise to find that large excesses of aqueous ammonia could be utilized at high temperatures and for extended periods of time while extracting substantially none of the Group VIII noble metal. The original theory as to the mechanism by which the Group VIII noble metal is redistributed on the zeolite by the ammonia treatment may hence be incorrect; if a soluble species of the noble metal is formed, it is apparently so highly basic that it is retained substantially quantitatively in the acid zeolite structure even in the presence of large excesses of aqueous ammonia.
However, in using the ammonium salt solution in the second step, the conditions of temperature and contact time must be suitably controlled because it is found that, in contrast to aqueous ammonia, the ammonium salts do tend to bring about a solubilization and leaching out of the Group VIII noble metal from the catalyst. This tendency can however be substantially avoided by controlling temperature and contact time.
Another progenitor of the present invention is disclosed in U.S. Pat No. 3,692,692, involving the sequential treatment of the damaged zeolites with aqueous solutions of ammonium salts, and with ammonia under the controlled conditions of hydration disclosed in the above noted application Ser. No. 321,670. The preferred techniques disclosed in said patent can achieve the same basic objectives as herein; however, it is found that the use of a volumetric excess of ammonium hydroxide solution brings about a more achieved and complete recovery of hydrogenation activity than can be achieve by using the mere adsorptive proportions disclosed in said patent. The term "volumetric excess" as used herein is intended to mean a volume sufficient to at least submerge the settled catalyst bed; i.e. sufficient to saturate the catalyst particles and fill the interstitial spaces therebetween.
Another progenitor of the present invention is disclosed in my copending application Ser. No. 344,880, filed Feb. 22, 1973, involving the use of a single solution of ammoniacal ammonium salt solution to effect rejuvenation. It has been found however that the sequential treatment disclosed herein results in greater overall recovery of activity, and with the preferred intermediate acid neutralization step, is substantially as economical as the single, mixed-solution technique of Ser. No. 344,880.